Some thoughts about riding on frosty mornings

[John D]

Following on from Claire's post about the frosty conditions this morning (Dec 22nd), some of this has been covered in previous years, but for those who may be new to the club I thought it worth posting again.

Like many of you (I suspect), the first site I consult on mornings like today is Roland's site which mirrors the School Based Weather Network. If you aren't checking it - you should be: http://figbug.com/weather/

The guy who runs the network, Ed Wiebe, works down the hall from me at UVic. Last year I asked if he could add dew point info to the site, which he did. When the air temperature falls below the dew point the water vapour in the air condenses onto solid surfaces. If this happens at temperatures above 0C you get dew, but if it happens at or below 0C then you get frost.

When I checked the website at about 4:30am this morning, the air temperatures were between 0.0-1.0C, and the corresponding dew points were hovering around zero. I decided that was too close for comfort and chose not to ride. A few things to bear in mind:

1. The dew point values reported on the school-based weather network are not "measured", per se. They are estimated from the measured air temperautre and the relative humidity.

2. The temperatures reported on the network are measured by instruments that are mounted on school roof-tops - often about 10m off the ground. Why is this importnat? Because the air temperatures at ground level, where we ride, can be a few degrees colder. This means that the risk of frost at ground level is higher than what one would conclude from looking at the reported temperatures and dew points.

3. We ride at 6:00am but today's sunrise wasn't until 8:04am. That means that the air temperatures continued to fall (and the risk of frost continued to rise) until at least sunrise. In fact, the coldest temperatures (and highest risk of frost) don't occur until almost an hour after sunrise.

Be safe out there!

P.S. Full disclsure - I'm an oceanographer, not a physicist. But if I've gotten any of this wrong, I trust that TSC's resident physics teacher (Eric S.) will correct me.

[EricS]

Right you are John. Fully agree with everything you said. Roland's site is awesome, but as you point out, if the dew-point is close to the temp, be careful. At 5:15 this morning, the dew-points were about 1 degree higher then the the temps in many locations, yet the ice covering my deck sent me back to bed. Good news: 3 hours more sleep, no broken bits, and a two hour ride in the basement with my daughter while watching Lord of the Rings (sprint when you see an orc, high cadence when Legolas says something obvious, you know the drill).

One thing people forget, is that despite having had the shortest day, the latest sunrises are still coming. From Dec 28th to Jan 3 sunrise is at 8:08 AM. Sunsets are getting later, faster, of course.

The air temp continually drops during the night as heat radiates away from the earth, but this continues even after the sun first rises, as the rate of energy coming in from the sun is still much less than the rate of energy loss due to outgoing radiation. This assumes no major changes in weather during this time interval.
We see this "thermal lag" on an annual scale too. This is why the coldest day of the year is not the shortest day. It's still likely 6 weeks away. And in the summer, late July/ early Aug is hotter than June 21st. Lag baby!

What's interesting to me, and something I have never given time to study, is that the rate of cooling seems to be higher during the first few minutes of sunrise. If I had to guess, I'd peg evaporative cooling. As the sun's rays strike liquid water on the ground (or in the air near the ground), it may cause the water to evaporate. This would draw in energy from the surrounding air and ground as it takes more energy to keep water in the gaseous state compared to the liquid state at the same temperature. Once the sun's rays are direct enough to provide this extra bit of energy, this effect would cease. Maybe 10 or 20 minutes? Combined with the above, maybe cooler for up to an hour after sunrise? Totally guessing here. Someday I'll research this, but right now it's easier to just guess, and drink rum and eggnog.

Eric

[AJ Neale]

Late season entry for post(s) of the year!

Thank you gentleman for illuminating ( no pun intended ) the risks and physics of early morning rides. You both sound more like physicists than the rest of us. I checked the Lansdowne Middle School temperature at 5:30 this morning only to find the last recorded temperature was 9:28 pm on Monday ........ we had a 3 hour power failure in the neighborhood! I decided to go old school and walk out to the street to see it glistening and came to the conclusion that it was cold. I also decided to ride, or at least meet up at Pareto and go straight to coffee. Thank you Claire!

Enjoy your rides on safe streets all!

Andrew

[John D]

I decided to go old school and walk out to the street to see it glistening and came to the conclusion that it was cold.

Evidence-based decision making is all the rage in policy making these days. Bravo!

[Robgrant]

So I was thinking we could use lasers to melt the frost in front of our wheels. Eric doesn't have any cool (hot) lasers, but I can get some. The wall plug efficiency of laser diodes is ~50-60%, water is nicely adsorbed by NIR lasers and the telecom industry has made some of these lasers cheap. We produce power on our bikes and could self power the lasers with pedalling and a generator. Will it work?

We ride right around freezing, so figure raising the temperature of the water 3-4 deg C would be enough. You need 4.8J/gxK, or about 12.5J or energy for 1g of water. But wait, there's more. This is peanuts compared to the phase change from solid to liquid. That requires 334J/g (according the the CRC Handbook of Chemistry and Physics). So really we are talking about ~350J of energy per gram of water to melt it.

It is reasonable to get a 1W NIR laser (say 980nm) in a compact package we could drive with batteries. The W is J/s, so we need at least 350W laser to melt 1g of ice in one second. If the wall plug efficiency is 50%, this means you would need to produce 700W @ the pedal- not my thighs.

Lasers are clearly not the answer to melt ice, so the conclusion is to stay in bed on icy days.